A certain Wheatstone bridge has the following resistor values: R1 = 10 kΩ, R2 = 720 Ω, and R4 = 2.4 kΩ. The unknown resistance is_____________?
A. 24 Ω
B. 2.4 Ω
C. 300 Ω
D. 3,000 Ω